1. Field of the Invention
The invention concerns a machine for filtering solid, especially fibrous, substances contained in suspension in industrial liquids to be treated.
2. Description of Related Art
It is a known fact that some types of industrial processes involve treatment waters which contain in suspension not only chemical agents but also solid compounds. Reference is made particularly to the tanning industry, wherein the industrial to be treated waters contain the hairs from the leather or to the wool-processing industry, also involving hairs and thin fibers. The same occurs in the foodstuff industry, wherein the washing and discharge water must be cleared of the fibrous particles which are present in the foodstuff to be treated.
One of the most widely employed methods for the elimination of these fibrous particles from the waters, prior to their chemical and biological treatment, consists in the use of large sedimentation installations comprising large volume tanks wherein the waste waters lose their speed, so that they allow the suspended solids to settle on the bottom. This method is rather costly, since the construction of these installations involves remarkable expenses and they also require large areas in order to obtain the desired results. Moreover, another limitation of this method arises from the fact that the removal of the sediments is rather difficult, since it can only be carried out after the tanks have been emptied or, worse still, the sediments are mixed with the water.
Another simpler and more effective method involves the use of machines equipped with revolving filters, into which the water to be treated, containing fibrous solid particles flows from a delivery chamber and an inlet raceway, which leads the water to be treated into another treatment chamber, wherein there are two vertically arranged discs facing each other and revolving around a horizontal axis which is perpendicular in relation to the flowing direction of the water to be treated. The discs are completed with filtering elements and the action of the water arriving in a direction which is pratically tangent to the surface of the filtering disc, combined with the revolving action of the disc, generates a resulting force which can be broken out according to a direction tangent to the disc and according to a direction normal to the disc.
The tangential component is practically ineffectual, while the normal component causes the water to flow through the filter, so that the filtering action takes place. During this process of the water flowing into the treatment chamber and during the rotation of the filtering discs a layer of deposited fibers builds up on the filtering surfaces, so that the holes through which the filtered water is discharged become partially clogged. Because of the revolution of the discs, the substances held by the filters between the two discs facing each other are constantly moved and turned over, so that said substances held by the filters slide on the surface of the filters and cause a self-cleaning action of the filter holes. Thus, the clogging of the filter holes is prevented and the filtering action is automatically maintained intact during the entire process. Obviously, as the filtering process progresses, the material exceeding a certain capacity of the tank is expelled through a discharge channel located on the wall of the treatment chamber between the two discs and practically near their center line.
The effect of the rotation of the discs carrying the filters is double.
In fact, on one hand a theoretical but real decrease of the dimensions of the holes for the passage of the discharge water is obtained and, as a consequence, causes an increase in the filtering performance because of the combined action of two factors. The first factor is represented by the fact that, because of the revolving movement of the filter, the filter hole "appears" smaller to the solid particles to be held back, since the rotation causes the window through which the solid particles would pass to appear smaller. The second factor is represented by the fact that the hole is slightly clogged by the layer of substances held back by the filter, so that, for instance, if the grids have holes measuring 250 microns, fibrous particles measuring 200 microns will be held back.
Another effect of the thus conceived and now mentioned machines is the fact that the filter is continuously self-cleaned by the layer of particles held between the filters.
Notwithstanding the considerable progress brought about by these machines in comparison with the sedimentation system, during the practical use a not negligible inconvenience has arisen. For example, when, for reasons depending on the industrial cycles, the amount of water to be treated increases so much, that the exceeding amount is drained through the overflow drainage, the water pressure between the filters decreases and creates whirlpools which cause the mass of substances which was held between the filters and acted as a filter-cleaning element to back up. Thus the machine comes to a standstill, since the self-cleaning effect is missing and it becomes necessary to restore the filtering process by adding some ballast material.